Implemented num_order methods on Integer & Rational numbers

src/indexing.rs

@@ -1116,8 +1116,9 @@ pub(crate) fn constant_key_alternatives(
         }
         */
         Literal::Integer(ref n) => {
-            if let Some(n) = n.to_isize() {
-                Fixnum::build_with_checked(n as i64).map(|n| {
+            let result = n.value().try_into();
+            if let Ok(value) = result {
+                Fixnum::build_with_checked(value).map(|n| {
                     constants.push(Literal::Fixnum(n));
                 }).unwrap();
             }